In the process of semiconductor manufacturing, lithography or photolithography is typically used to project light through a reticle exposing a silicon wafer coated with photosensitive resist in select regions to define circuit elements. An illumination system has been used in step-and-scan photolithography equipment sold under the trademark MICRASCAN by SVG Lithography Systems, Inc. Wilton, Conn. In this photolithography equipment, the reticle and the wafer move with different speeds. The different speeds having a ratio equal to the magnification of the projection optics. A rectangular or slit field defined by the illumination system is scanned over the reticle and over the wafer. A vertical field delimiter frames the vertical field height, and horizontal framing blades frame the horizontal field width. It is desirable to have as uniform an exposure field as possible. The illumination level is the integral, in the scan direction, of the illumination on the wafer. Often the illumination is not uniform enough. To obtain a uniform exposure or a constant level of illumination from the top to bottom longitudinally along the rectangular exposed field, an adjustable slit is often required. In the past adjustable slits have been used that employed a line of nails or projections perpendicular to the illumination beam. Individual nails or projections would be pushed into the illumination beam to make more uniform the illumination level or energy. Additionally, metal strips placed at an angle to the illumination beam would be bent or warped by rods thereby adjusting and making more uniform the illumination level or energy. One type of compliant member or adjustable slit is disclosed in U.S. Pat. No. 4,516,852 entitled "Method and Apparatus for Measuring Intensity Variations in a Light Source", issuing to Liu et al on May 14, 1985. Therein disclosed is an arcuate slit that is adjusted with a deformable band. While these prior adjustable slit devices have been helpful in providing a more uniform illumination field the ever increasing demands placed on lithography in reducing feature size of semiconductor devices and increasing yield require an even more uniform illumination field.